press a key, timing sequence?

My experiment could change my game entirely!

My plan is to have this rifle which of futuristic design.. etc, If this rifle could have a 'charging' effect. Here is a better example of what the idea flow chart goes.

Player equips weapon---> Player ,if (Input.GetKeyDown("1"))----> initiates timing sequence---> every 5 seconds Gui Texture on rifle changes----> after 20 seconds rifle is ready to fire--- unlock the function of firing EXAMPLE= if (Input.GetButtonDown("Fire1"))This function is enabled---------> FIRE-----Restart